There are two things you could be referring to; the first is a sustain pedal for a keyboard, which is basically a simple momentary on/off footswitch that activates the sustain feature in your keyboard. This is most likely not what you are referring to.

The other pedal you could be referring to is a guitar effect. Boss makes a pedal called the CS-3 (I think) Compression/Sustainer. The sustain feature of this pedal boosts the volume of the sound from your guitar as the actual signal from the pickups fades, which results in each note maintaining an even volume up until the signal drops below a certain threshold. This makes it seem like your notes are sustaining for longer than usual.

OK, someone do a better job of explaining this!